1. Field of the Invention
The present invention relates to an optoelectronic element module and more particularly, to an optoelectronic element module having an optoelectronic element such as a laser diode and a Peltier device for cooling the element, both of which are encapsulated in a package.
2. Description of the Prior Art
FIG. 1 shows a conventional laser diode module applicable to the optical communication field.
As shown in FIG. 1, this conventional laser diode module has a laser diode assembly, a lens holder assembly, and a Peltier unit 106, which are encapsulated in a package 105. These two assemblies are located on the Peltier unit 106.
The package 105 is formed by a flat base plate 105a on which the Peltier unit 106 is placed, a sidewall 105b fixed onto the base plate 105a to surround the laser diode and lens holder assemblies and the Peltier unit 106, and a flat cap 105c fixed to the sidewall 105b. The bottom end of the sidewall 105b is connected to the base plate 105a. The cap 105c is connected to the top end of the sidewall 105b. The inner space of the package 105 has a shape of an approximately rectangular parallelepiped.
The Peltier unit 106 serves to cool the laser diode 101 on operation. The Peltier unit 106 is comprised of a lower flat plate 106b, an upper flat plate 106c parallel to the lower plate 106b, and a plurality of pairs of Peltier devices 106a regularly arranged along the lower and upper plates 106b and 106c between these plates 106b and 106c. The pairs of Peltier devices 106a are sandwiched by the lower and upper plates 106b and 106c.
The laser diode assembly includes a laser diode (i.e., semiconductor laser) 101, a heat sink 102, a thermally conductive carrier 103, and a photodiode 104.
The laser diode 101 is mounted on the carrier 103 through the heat sink 102 for sinking the heat generated in the diode 101 on operation. The laser diode 101 serves as a light source for emitting a predesigned output light beam.
The photodiode 104 is mounted directly on the carrier 103 at the back of the laser diode 101. The photodiode 104 serves to monitor the output light beam emitted from the laser diode 101.
The carrier 103 serves to carry or support the laser diode 101, the heat sink 102, and the photodiode 104. The carrier 103 is fixed onto the upper plate 106c of the Peltier unit 106 by a solder layer 112a made of a solder such as BiSn or InPbAg.
The lens holder assembly includes a lens holder 111, an optical lens 107, an optical isolator 108, a slide ring 109, and a pigtail 110.
The lens holder 111 has an approximately cylindrical shape and is mounted on the upper plate 106c of the Peltier unit 106 in front of the laser diode 101 by the solder layer 112a. The rear end of the lens holder 111 is engaged with the carrier 103.
The optical lens 107 and the optical isolator 108 are held in the inside of the lens holder 111 to be located on its longitudinal axis. The lens 107 is located at the back end of the holder 111 in the vicinity of the laser diode 101. The lens 107 serves to collect the output light beam emitted from the laser diode 101.
The isolator 108 is located at approximately the middle of the holder 111 apart from the lens 107. The isolator 108 serves to prevent reflected light of the output light beam from entering the pigtail 110.
The pigtail 110 is formed by a short piece of an optical fiber. An inner end of the pigtail 110 is fixed to an opposing front end of the lens holder 111 by the slide ring 109. The slide ring 109 is fixed to the holder 111 and the pigtail 110 by spot welding using an Yttrium-Aluminum-Garnet (YAG) laser. The pigtail 110 extends along the longitudinal axis of the lens holder 111 and the base plate 105a of the package 105. An outer end (not shown) of the pigtail 110 protrudes from the package 105 through a window 105d of the package 105. An optical fiber (not shown) is connected to the outer end of the pigtail 110 outside the package 105 as necessary. The window 105d is formed in the sidewall 105b. The pigtail 110 is fixed by a solder material 115 such as indium (In).
The lower plate 106a of the Peltier unit 106 is fixed to a mounting surface 105aa of the package 105 by a solder layer 112b. The mounting surface 105aa is formed on the upper surface of the base plate 105a of the package 105. This solder layer 112b is made of a solder such as BiSn or InPbAg.
The output light beam, which is emitted from the laser diode 101 on operation, passes through the lens 107 and the isolator 108 to enter the pigtail 110 at its inner end. The output light beam is transmitted by an optical fiber (not shown) connected to the outer end of the pigtail 110.
The heat generated by the laser diode 101 on operation is propagated to the Peltier unit 106 through the heat sink 102 and the carrier 103. Then, the heat is effectively transmitted by the Peltier unit 106 to the base plate 105a of the package 105, thereby cooling the laser diode 101 on operation. The heat transmitted to the base plate 105a is automatically radiated away through a board (not shown) on which the laser diode module is mounted.
When the conventional laser diode module in FIG. 1 is used as an excitation or driving source for an optical fiber amplifier, a high optical output needs to be taken out from this module stably. In this case, a driving current for the laser diode 101 is required to be set as large as approximately 500 mA. Such the large driving current usually raises the temperature of the laser diode 101 on operation by approximately 30 degrees in centigrade.
To keep the normal operation of the laser diode 101 stable at such the raised temperatures, therefore, the Peltier unit 106 is necessary to be large-sized for the purpose of large cooling capacity. This leads to a wider heat-transmission area of the lower plate 106b of the Peltier unit 106 to the base plate 105a of the package 105 than popular laser diode modules designed for optical communication.
Thus, the conventional laser diode module shown in FIG. 1 has the following problem.
Because of the wide heat-transmission area between the Peltier unit 106 and the package 105, this conventional laser diode module is unable to withstand an environmental test including abrupt temperature change such as an ON/OFF test of the Peltier unit 106. As a result of this, there is a problem that this conventional module has insufficient reliability from the viewpoint of temperature change.
The reason is that the Peltier unit 106 is destroyed or damaged due to thermal stress caused by the difference between the thermal expansion coefficients of the lower plate 106b of the Peltier unit 106 and the base plate 105a of the package 105, thereby eliminating or degrading the predesigned cooling performance of the Peltier unit 106.
A known solution to this problem is disclosed in the Japanese Non-Examined Patent Publication No. 62-276892 published in December 1987. In this solution, the lower plate 106c of the Peltier unit 106 is approximated in thermal expansion coefficient to the base plate 105a of the package 105 by selecting the materials. However, the following problem will occur.
Specifically, since the lower and upper plates 106b and 106c of the Peltier unit 106 are usually made of a ceramic such as aluminum nitride (AlN) and alumina (Al.sub.2 O.sub.3), the package 105 needs to be made of a metal such as Fe--Ni--Co alloy termed "kovar" or CuW alloy containing 20% copper (i.e., CuW-20 alloy) in order to approximate its thermal expansion coefficient to the ceramic. However, these alloys have thermal expansion coefficients as low as approximately 0.5 cal/cm.cndot.sec.cndot.deg. Thus, it is difficult for the above-identified known solution disclosed in the Japanese Non-Examined Patent Publication No. 62-276892 to ensure a desired cooling performance at a driving current of approximately 500 mA.
Another known solution to the above-identified problem relating to insufficient reliability is disclosed in the Japanese Non-Examined Utility-Model Publication No. 4-63669 published in May 1992. In this solution, the package 105, the lower and upper plates 106c and 106b of the Peltier unit 106, and the carrier 103 are approximated in thermal expansion coefficient to each other. However, the same problem as the solution disclosed in the Japanese Non-Examined Patent Publication No. 62-276892 will occur.